Nosema ceranae is an emergent and potentially virulent pathogen of the honey bee (Apis mellifera) that has spread across the world in the last 10 or so years. Its precise origin and timing of spread are currently unclear because of a lack of appropriate genetic markers and inadequate sampling in putative Asian source populations. Though it has been dismissed as a cause of CCD in the USA based on correlational analyses of snapshot sampling of diseased hives, observations of naturally infected colonies suggest that it leads to colony collapse in Spain. Experiments are sorely needed to investigate its impact on individuals and colonies, and to pin down a causal relationship between N. ceranae and colony collapse. Whether N. ceranae is displacing N. apis is uncertain. For temperate zone apiculturalists, global climate change may mean that N. ceranae presents more of a challenge than has hitherto been considered the case.